1. Technical Field
The present invention generally relates to low viscosity marine cylinder lubricating oil compositions.
2. Description of the Related Art
In the not so distant past, rapidly escalating energy costs, particularly those incurred in distilling crude oil and liquid petroleum, became burdensome to the users of transportation fuels, such as owners and operators of seagoing ships. In response, those users have steered their operations away from steam turbine propulsion units in favor of large marine diesel engines that are more fuel efficient. Diesel engines may generally be classified as slow-speed, medium-speed, or high-speed engines, with the slow-speed variety being used for the largest, deep shaft marine vessels and certain other industrial applications.
Slow-speed diesel engines are unique in size and method of operation. The engines themselves are massive, the larger units may approach 200 tons in weight and an upward of 10 feet in length and 45 feet in height. The output of these engines can reach as high as 100,000 brake horsepower with engine revolutions of 60 to about 200 revolutions per minute. They are typically of crosshead design and operate on the two-stroke cycle.
Medium-speed engines, on the other hand, typically operate in the range of about 250 to about 1100 rpm and may operate on either the four-stroke or the two-stroke cycle. These engines can be of trunk piston design or occasionally of crosshead design. They typically operate on residual fuels, just like the slow-speed diesel engines, but some may also operate on distillate fuels that contain little or no residue. These engines can also be used for propulsion, ancillary applications or both on deep-sea vessels.
Slow- and medium-speed diesel engines are also extensively used in power plant operations. A slow- or medium-speed diesel engine that operates on the 2-stroke cycle is typically a direct-coupled and direct-reversing engine of crosshead construction, with a diaphragm and one or more stuffing boxes separating the power cylinders from the crankcase to prevent combustion products from entering the crankcase and mixing with the crankcase oil. The notable complete separation of the crankcase from the combustion zone has led persons skilled in the art to lubricate the combustion chamber and the crankcase with different lubricating oils.
Accordingly, in large diesel engines of the cross-head type used in marine and heavy stationary applications, the cylinders are lubricated separately from the other engine components. The cylinders are lubricated on a total loss basis with the cylinder oil being injected separately to quills on each cylinder by means of lubricators positioned around the cylinder liner. Oil is distributed to the lubricators by means of pumps, which are, in modern engine designs, actuated to apply the oil directly onto the rings to reduce wastage of the oil.
The high stresses encountered in these engines and the use of residual fuels creates the need for lubricants with a high detergency and neutralizing capability even though the oils are exposed to thermal and other stresses only for short periods of time. Residual fuels commonly used in these diesel engines typically contain significant quantities of sulfur, which, in the combustion process, combine with water to form sulfuric acid, the presence of which leads to corrosive wear. In particular, in two-stroke engines for ships, areas around the cylinder liners and piston rings can be corroded and worn by the acid. Therefore, it is important for diesel engine lubricating oils to have the ability to resist such corrosion and wear.
Accordingly, a primary function of marine cylinder lubricants is to neutralize sulfur-based acidic components of high-sulfur fuel oil combusted in slow-speed 2-cycle crosshead diesel engines. This neutralization is accomplished by the inclusion in the marine cylinder lubricant of basic species such as metallic detergents. Unfortunately the basicity of the marine cylinder lubricant can be diminished by oxidation of the marine cylinder lubricant (caused by the thermal and oxidative stress the lubricant undergoes in the engine), thus decreasing the lubricant's neutralization ability. The oxidation can be accelerated if the marine cylinder lubricants contain oxidation catalysts such as wear metals that are generally known to be present in the lubricant during engine operation.
Typically, marine cylinder lubricants for use in marine diesel engines have a viscosity in the range of 16.5 to 25 centistokes (cSt) at 100° C. In order to formulate such a lubricant, a brightstock is combined with a low viscosity oil, e.g., an oil having a viscosity from 4 to 6 cSt at 100° C. However, supplies of bright stock are dwindling and therefore bright stock cannot be relied upon to increase the viscosity of marine cylinder lubricants to the range of 16.5 to 25 cSt at 100° C. that manufacturers recommend. In addition, Hart's Lubricant World, September 1997, pp. 27-28, (referenced in EP 1967571) discloses that “Due to low-operating speeds and high loads in marine engines, high viscosity oils (SAE 40, 50, and 60) typically are required. Because hydrocracking results in a viscosity loss of the base stocks, marine oils cannot generally be formulated solely with hydrocracked base stocks, but require the use of significant amounts of bright stock. However, the use of bright stock is not desirable because of the presence of oxidatively unstable aromatics.”
One solution to this problem is to use thickeners such as polyisobutylene or viscosity index improver compounds such as olefin colopymers to thicken the marine cylinder lubricants. However, these materials add to the cost of the marine cylinder lubricants. Another solution is to use lower viscosity marine cylinder lubricants; but the wear performance of low viscosity MCLs has not been well investigated.
European Patent Application Publication No. EP 1 967 571 (“the '571 application”) discloses a liquid lubricant base oil composition useful in marine engine applications, such as in 2-stroke marine diesel engine cylinder oils, 2-stroke marine diesel engine system oils, and 4-stroke marine diesel engine crankcase lubricants. The lubricant base oil composition disclosed in the '571 application contains (a) a base stock comprising at least 95 wt. % saturated hydrocarbons, and (b) 0.2 to 30 wt. % of an aromatic (bright stock) extract. The '571 application further discloses that the liquid lubricant base oil composition has a viscosity in the range of 7 to 40 cSt at 100° C. In addition, the '571 application discloses that the combination of a Group II base oil and a low polycyclic aromatic brightstock extract demonstrated improved viscosity ratio and improved oxidation and wear performance.
Applicants have found that wear performance of a marine cylinder lubricant used in a cylinder of a 2-stroke crosshead marine diesel engine can be maintained by formulating a marine cylinder lubricant containing a major amount of a basestock selected from the group consisting of a Group II basestock, a Group III basestock and mixtures thereof, wherein the marine cylinder lubricant has a kinematic viscosity at 100° C. of from 13 to about 16.2 cSt; and contains less than about 10 wt. % bright stock.